Abstract
The closure of the Isthmus of Panama (about 3.1 million years ago) separated previously continuous populations and created two groups of extant species, which live now in the Pacific and Atlantic drainage systems. This relatively recent event was a trigger to diversification of various species in the Neotropics, nonetheless there are exemplars that do not show sufficient morphologic variability to separate them by traditional morphological tools. About 60 years ago, some freshwater decapod species with high morphological similarity were separate by previous researchers, based on geographical distribution, in Pacific and Atlantic and considered as “sister species”. However, the complete isolation of these prawns by this geographical barrier is questionable, and it has generated doubts about the status of the following transisthmian pairs of sibling species: Macrobrachium occidentale × Macrobrachium heterochirus, Macrobrachium americanum × Macrobrachium carcinus, Macrobrachium digueti × Macrobrachium olfersii, Macrobrachium hancocki × Macrobrachium crenulatum, Macrobrachium tenellum × Macrobrachium acanthurus and Macrobrachium panamense × Macrobrachium amazonicum. Here we evaluated the relation among these pairs of sibling species in a molecular phylogenetic context. We generated 95 new sequences: 26 sequences of 16S rDNA, 25 of COI mtDNA and 44 of 18S nDNA. In total, 181 sequences were analyzed by maximum likelihood phylogenetic method, including 12 Macrobrachium transisthmian species, as well as seven other American Macrobrachium species, and two other palaemonids. Our analysis corroborated the morphological proximity of the sibling species. Despite the high degree of morphological similarities and considerable genetic diversification encountered among the transisthmian sister species, our data support the conclusion that all species included in sibling groups studied herein are valid taxonomic entities, but not all pairs of siblings form natural groups.
Highlights
In the late Pliocene, the closure of the Isthmus of Panama was a trigger to the diversification of many species in the Neotropics
Considering the doubt whether the previously indicated species of Central American Macrobrachium are sister taxa or not, our study aimed to evaluate in a molecular phylogenetic context the relationships among 12 transisthmian Macrobrachium “sibling species” from the Americas in order to assess the validity of their current species level
The topology obtained by maximum likelihood from concatenated genes (16S, 18S and COI) analyses confirmed that the transisthmian sibling species (M. heterochirus × M. occidentale – Sibling 1, M. carcinus × M. americanum – Sibling 2, M. olfersii × M. digueti – Sibling 3, M. crenulatum × M. hancocki – Sibling 4, and M. acanthurus × M. tenellum – Sibling 5) are closely related by well-supported clades (Fig. 1)
Summary
In the late Pliocene, the closure of the Isthmus of Panama was a trigger to the diversification of many species in the Neotropics. The separation of previously continuous populations created two groups of extant species, which live in the Atlantic and Pacific drainage systems This vicariant event opened a unique opportunity for studies on evolution, divergence and speciation processes (Knowlton et al 1993, Knowlton and Weigt 1998, Lessios 2008). In spite of the geographic separation, some species are difficult or impossible to distinguish using traditional morphological features, and are called “sibling species” (see Knowlton 1993 and references cited therein). These sibling species refer to pairs of species that are genetically closely related, but reproductively isolated (Mayr 1963, Steyskal 1972, Knowlton 1986). A pair of species, reproductively isolated and very similar in morphology, is not necessarily considered as sibling species, and an interdisciplinary approach is necessary to evaluate this conclusion
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